Generally, EVOH is excellent in transparency, gas barrier property, aroma-retaining property, solvent resistance, oil resistance and other characteristics. Owing to such characteristics, EVOH is molded into and utilized as films or sheets and bottles or other containers, among others, for use as food packaging materials, drug packaging materials, industrial chemical packaging materials or agrochemical packaging materials, for instance.
As a method of producing EVOH pellets, a process is known which comprises extruding an alcohol solution or a solution in an alcohol-water mixed solvent of EVOH as obtained by copolymerizing an ethylene and vinyl acetate and further saponifying the thus-obtained ethylene-vinyl acetate copolymer into a coagulation liquid in the form of a strand and cutting the resulting strand to give EVOH pellets.
The EVOH pellets obtained in the above manner are generally subjected to drying treatment to give product pellets. As regards the method of drying on that occasion, Japanese Patent Publication S46-37665 suggests that EVOH be subjected to fluidized-state drying with stirring in an inert gas atmosphere with an oxygen content of not more than 5% at 95.degree. C. or below.
The thus-obtained product pellets are molded into various kinds of product. In their molding, melt molding is generally conducted, whereby they are processed into such forms or shapes as films, sheets, bottles, cups, tubes, pipes and so forth. The processability (moldability) on that occasion is very important and, in many instances, generally for the purpose of providing mechanical strength, moisture resistance, heat sealability and other properties, they are coextruded with a substrate such as a polyolefin resin via an adhesive layer to give laminates.
For improving the moldability mentioned above, the method comprising incorporating a boron compound in EVOH (Japanese Kokai Tokkyo Koho S59-192564 and S55-12108, Japanese Patent Publication S49-20615, etc.) and the method comprising causing EVOH to contain a metal salt such as sodium acetate (Japanese Kokai Tokkyo Koho S51-91988, S56-41204 and S64-66262, etc.) have been proposed. Furthermore, it has been attempted to incorporate a phosphoric acid compound in EVOH (Japanese Kokai Tokkyo Koho S52-954 and H02-235952, etc.), and the present applicant has also proposed to treat EVOH with a phosphoric acid compound to improve the thermal stability and/or melt moldability (e.g. prevention of fish eyes or gels) (Japanese Kokai Tokkyo Koho S62-143954).
However, in the above-mentioned methods of producing EVOH pellets, the weight ratio between the coagulating liquid and the EVOH strand (coagulant/strand ratio) is set at a low level not higher than 50 so that the amount of solvent may be reduced and the loss of EVOH due to dissolution may be prevented. It has been revealed, however, that there are drawbacks in that case; for example, the strand may break or a poor pellet size precision may result. Therefore, when those EVOH pellets obtained by the conventional methods are used in extrusion molding, the charge into the extruder and the load on the extruder, for instance, tend to fluctuate and, as a result, it becomes not always easy to perform the molding procedure in a stable manner. Therefore, EVOH pellets uniform in shape and size are desired.
Further, as regards the method of drying EVOH, it has been revealed that the fluidized-state drying described in Japanese Patent Publication S46-37665 as such, though effective in reducing fish eyes of EVOH, allows great fluctuations in torque and discharge rate in the step of melt molding and further may cause defects in uniformity of thickness of moldings. Consumers are now strongly demanding marketing of EVOH pellets excellent in such melt moldability.
In addition, to cope with recent increasing requirements with regard to performance characteristics of moldings, the technologies of the above-cited Japanese Kokai Tokkyo Koho S59-192564 and S55-12108, Japanese Patent Publication S49-20615, and Japanese Kokai Tokkyo Koho S51-91988, S56-41204, S64-66262, S52-954 and H02-235952 were carefully evaluated and, as a result, it was revealed that these technologies, when applied to melt molding of EVOH pellets, indeed produce improvements with respect to fish eyes or gels with a diameter not less than 0.1 mm but cannot always solve the problem of small fish eyes or gels with a diameter less than 0.1 mm and that, in particular, said technologies fail to give thorough consideration to the melt moldability in the production of multilayer laminates and possibly allow occurrence of fish eyes and the like smaller than 0.1 mm in size on multilayer laminates depending on the conditions of molding thereof. It was thus found that further improvements are desirable to solve these problems, among others.
It is an object of the present invention to provide a method of producing EVOH pellets by which the occurrence of fish eyes and the like smaller in size than 0.1 mm can be prevented in particular in producing multilayer laminates and by which good long-run moldability can be attained.